1. Field of the Invention
The present invention relates to improvements in methods and apparatus for automatically spreading or "smearing" a fluid specimen on a microscope slide for subsequent analysis. The invention is particularly useful for producing a blood smear on a microscope slide for blood cell analysis.
2. Discussion of the Prior Art
Microscopic examination of biological fluids (e.g. blood, urine, etc.) on glass slides is an old and well-known technique used for diagnosing and evaluating a patient's health. Examination of a smeared sample of fluid on a glass slide often permits a physician or clinician to more accurately analyze a fluid sample and to diagnose a variety of fluid-related disorders.
To conduct such observational analysis of fluid samples, it is highly desirable that the slide be prepared in such a manner that a substantially uniform "monolayer" of fluid cells is formed on at least a portion of one of the slide's planar surfaces. A monolayer assures that individual cells can be examined without being masked by overlying cells. Having produced the desired monolayer, the examining clinical or researcher can count the number of cells of different types and/or examine the cell morphology.
Presently, the most commonly used method of producing an examinable blood ssmear on a glass microscope slide is a manual method in which two slides are used. Normally, such slides are approximately 3 inches (75 mm) in length, 1 inch (25 mm) in width and 0.04 inches (1 mm) in thickness. According to the such method, the technician first deposits a drop of blood on the planar surface of one slide and then uses an edge (usually the shorter edge) of the second slide to smear the blood drop on the drop-supporting surface of the first slide. Smearing of the drop is effected by positioning the smearing slide at an acute angle relative to the drop-supporting slide, sliding the smearing slide in a first direction so that the underlying planar surface of the smearing slide contacts the blood drop and allows the drop to wick, due to capillary action, across the width of the smearing slide, and then sliding the smearing slide in the opposite direction, thereby pulling a blood film across the drop-supporting slide by capillary forces. A stain may then be applied to the blood film (usually referred to as a "smear") in order to facilitate the differentiation of the various types of cells.
A problem with the manual technique for producing microscopic slides of blood smears is that there is often a lack of uniformity in the appearance of the smears. This is true even When the same clinician or researcher has prepared all the smears being examined at a given time. As may be appreciated, there are many variables in making a blood smear on a glass slide, including, for example, the volume of the blood drop being smeared, the angle of the smearing member relative to the drop-supporting surface, the rate at which the smearing member moves across the drop-supporting slide, the pressure between the smearing member and the slide, etc. This is further complicated by the fact that the ability to prepare good quality slides varies with the cellular make-up of the blood sample, the surface characteristics of the slide, the edge characteristics of the smearing member and the skill of the slide preparer. In view of these multiple variables, a substantial fraction of blood slides made manually are of little or no use. Unacceptable blood smears include those exhibiting (a) a skewed distribution of cells across the width of the slide, as produced, e.g., by a non-uniform pressure being applied to the smearing edge, and (b) multiple parallel streaks in the blood smear, as produced, e.g., by edge irregularities in the smearing edge of the smearing slide, and (c) smear thickness variations measured in the direction of the smear, as produced, e.g., by irregularities in the frictional characteristics of the drop-supporting planar surface of the slide receiving the smear.
One solution to some of the above-noted problems associated with the manual production of blood smear slides is to provide a device which automates the steps of the manual process. One such device is disclosed, for example, in U.S. Pat. No. 5,209,903, issued in the name of Kanamori, et al. This patent discloses a blood analyzing system which incorporates a conveyor for handling and transporting vials of blood through a blood analyzer and, subsequently, through an automatic blood smear generator. The latter operates to automatically aspirate a blood sample from a vial, deposit a drop of blood on a slide, spread the blood drop to produce a blood smear on the slide, dry the blood smear, print patient information on the slide, and then deposit the slide in a basket adapted for use in a staining bath. Spreading of the blood drop on the slide is effected by the edge of a smearing glass which is used repeatedly to produce multiple slides. Between the making of successive slides, the smearing glass is cleansed by a suitable washing device. While the blood smear is made, the smear-receiving glass slide is maintained in a horizontal position atop a pair of conveyor belts. The rate of movement of the smearing glass, the volume of the blood drop, and the angle of the smearing glass relative to the horizontally oriented drop-supported slide are all variable and operate under the control of a suitable controller which responds to the output of the blood analyzer(s) located upstream of the slide maker along the conveyor belt.
While the automatic slide-making device disclosed in the above-mentioned patent eliminates some of the variables associated with the manual approach to slide making, such device can be problematic in several respects. For example, since the glass smearing member is used repeatedly to make successive slides, apparatus must be provided for cleaning the smearing member between successive slides. Further, since the same smearing member is used to make a large number of slides, any irregularities in the smearing edge of such member can result in streaks which will be exhibited in all slides made by such member. Moreover, since no special care is taken to compensate for any surface irregularities in the smear-receiving glass, variations in thickness of the smear can arise.